In U.S. Pat. No. 6,888,196, a conventional structure with trenched source-body contact structure of a trench MOSFET is disclosed, as shown in FIG. 1, wherein an N-channel trench MOSFET comprising a plurality of trenched gates 110 surrounded by n+ source regions 112 encompassed in P body regions 114 is formed in an N epitaxial layer 102 over an N+ substrate 100 coated with back metal 190 as drain. To connect said source regions 112 and said body regions 114 to a source metal 122, a trenched source-body contact 118 is employed penetrating through a contact interlayer 120, said n+ source regions 112 and extending into said P body regions 114. Furthermore, a p+ body ohmic contact doped region 116 is implanted surrounding bottom of said trenched source-body contact 118 to decrease a contact resistance between said P body regions 114 and said trenched source-body contact 118.
The conventional structure in FIG. 1 is encountering a technical difficulty which is that the trenched source-body contact 118 causes significant reduction in output capacitance Coss as explained below, thus impacts DC-DC application issue. However, in the DC-DC application, PWM (Pulse-Width Modulation) IC can not match well with the device with low Coss, hence resulting in low efficiency.
As we all know that, in a trench MOSFET, the output capacitance Coss=Cds+Cgd, where Cds is capacitance between drain and source, and Cgd is capacitance between gate and drain, as illustrated in FIG. 1. The Coss reduction mentioned above is mainly due to the Cds reduction as result of reduction in mesa area between two adjacent trenched gates when trench width of the trenched source-body contact structure is shrunk. Therefore, in order to increase the Coss, it is a key point to increase the Cds which is proportional to Ba/Dw, where Ba is area of body region interfaced with the epitaxial layer between two adjacent trenched gates and Dw is total depletion width in the body region and the epitaxial layer at the interface which is inversed to doping concentration of the body region and the epitaxial layer at the interface.
Accordingly, it would be desirable to provide new and improved power semiconductor devices to avoid the constraint discussed above.